The size of the loading dose is a function of body weight - see below
Unfortunately some doctors persist in using Vitamin D2 instead of D3, which has been shown to have many problems

Loading may be done as quickly as a single day, to as slowly as 3 months.
It appears that spreading the loading dose over 4-20 days is a good compromise - not too fast, not too slow.

Loading is typically oral, but sometimes by injection (I.M,)

The loading dose persists in the body for about 3 months
The loading dose should be followed up with continuing maintenance - typically 4,000 to 10,000 IU daily average
Unfortunately many doctors fail to follow-up with the maintenance dosing.

As about 1 in 300 people have some form of mild allergic reaction to vitamin D supplements,
it appears prudent to test with a small amount of the exact form of vitamin D before giving a loading dose
Causes for a mild allergic reaction appear to be: (in order of occurance)
1) lack of magnesium - which can be easily added
2) allergy to capsule contents - oil (powder does not appear to cause any reaction)
3) allergy to the tiny amount of D3 itself (allergy to wool) ( alternate: D3 made from plants )

CONCLUSION: The cholecalciferol loading dose required to reach the serum 25-OHD(3) target level of 75 nmol/l can be calculated as follows:
dose (IU)=40x(75 - serum 25-OHD(3))xbody weight.CLICK HERE for PDF of the study and equationEquation converted into units of ng and pounds Dose to get 30 ng (in ng and pounds) = 35 x (30 - serum 25-OHD(3)) x body weight.Typically spread over over 8 weeks
A very commonly prescribed loading dose in the US is 50,000 IU per week for 8 weeks, for a total of 400,000 IUExample uses of the equation to achieve 30 ng (on average)